Method for the production of cast steel strip

ABSTRACT

A method for the production of cast steel strip in a continuous procedure includes casting a steel melt into a casting gap having longitudinal sides of which are formed by walls that move during the casting process, to form the steel strip, and holding the steel melt, which is present above the casting gap in a melt pool, under an atmosphere containing nitrogen and hydrogen. This method allows the production of high-quality steel strips having a significantly improved surface composition compared to the prior art in that the hydrogen content of the atmosphere is greater than 0 mol % to 10 mol %, and the Cr, Mo, Nb, Si, Ti, Ni, Mn, C or N contents of the cast steel melt, which are selectively present in each case for adjusting the characteristics of the steel strip, are in each case selected so that a ratio Cr eq /Ni eq ≧1.7.

The invention relates to a method for the production of cast steelstrip, wherein, in a continuous procedure, a steel melt is cast into acasting gap, the longitudinal sides of which are formed by walls thatmove during the casting process, to form the steel strip, and the steelmelt, which is present above the casting gap in a melt pool, is heldunder an atmosphere containing nitrogen and hydrogen. A method of thistype is described, for example, in EP 0 409 645 B1.

A fundamental problem in the production of steel strip cast directlyfrom a steel melt is that of conducting the casting process in such away that an optimal, high-quality strip surface is obtained. Inpractice, it has thus been found that significant surface defects arecaused by the formation of cracks and the entrapment of oxides (“scums”)in the regions of the cast strip in proximity to the surface. In manycases, cast strips produced by strip-casting also conventionally have aheterogeneous surface structure, resulting in non-uniformity of theperceptible shine of the finished cast strip. A “heterogeneous shine” ofthis type is a further quality defect restricting the saleability ofcast strip.

The aforementioned EP 0 409 645 B1 proposes, for improving the surfacecharacteristics of cast strip, forming dimples in the elementsdelimiting the casting gap in the device used for casting the strip,which elements move during the casting operation. In addition, a gasmixture is to be blown into the area of what is known as the “meniscus”,at which the melt enters into contact with the dimples in the movingelements. The gas mixture is to contain 30% by volume to 90% by volumeof a gas soluble in metal. An insoluble gas may also be provided.

EP 0 409 645 B1 cites N₂, H₂, CO₂, CO and NH₄ as gases soluble in metalsuitable for carrying out the known method. Argon and helium are givenas examples of an insoluble gas. However, only the effects of gasmixtures comprising N₂ and Ar contents are described specifically inthis regard.

According to EP 0 409 645 B1, the effect of the gas mixture blown intothe meniscus area in combination with the dimples formed in the movingelements, which delimit the casting gap, is that any air present in thedimples and gases issuing from the cast metal are expelled and theblown-in gas enters the dimples to replace them. Subsequently in theprocess, the melt enters the dimples. As a result of its solubility, thegas located therein is absorbed by the melt, so the melt may enter thedimples unimpeded. A temporary interlocking linkage, which ensures thatthere is no relative movement between the elements and the solidifiedshell formed as a result of the solidification of the melt on the movingelements, may thus be achieved between the solidifying melt and themoving elements. According to the information provided in EP 0 409 645B1, such relative movement causes the formation of surface cracks.

EP 0 409 645 B1 also proposes protecting from oxidation the melt presentabove the casting gap in what is known as the “melt pool” in that themelt pool is held under a non-oxidising atmosphere. It is regarded asbeing preferable if the atmosphere is formed from a nitrogen/argon gasmixture.

What are known as twin-rollers, in which the elements, which delimit thecasting gap at the longitudinal sides thereof and move during thecasting operation, are in the form of casting rollers, which are rotatedin opposite directions and cooled during the casting operation, areconventionally used for strip-casting. A high degree of technicalcomplexity, which complicates the regularly required maintenance of thecasting rollers, is necessary for forming the dimples required accordingto EP 0 409 645 B1, for example, in the surfaces of the casting rollersof a twin-roller.

Moreover, practical tests have revealed that the problem of theproduction of surface defects owing to the formation of cracks andentrapment of oxides may not be solved even if the surface of the meltpool present above the casting gap is sealed off against atmosphericoxygen, as advocated in EP 0 409 645 B1, by a non-oxidising atmosphere.

The object of the invention was accordingly to specify a processallowing the production of high-quality steel strips having asignificantly improved surface composition compared to the prior art.

Starting from the above-described prior art, this object has beenachieved by a method for the production of cast steel strip, wherein, ina continuous procedure, a steel melt is cast into a casting gap, thelongitudinal sides of which are formed by walls that move during thecasting process, to form the steel strip, and the steel melt, which ispresent above the casting gap in a melt pool, is held under anatmosphere containing nitrogen and hydrogen, wherein, according to theinvention, the hydrogen content of the atmosphere is greater than 0 mol% to 10 mol %, and the Cr, Mo, Nb, Si, Ti, Ni, Mn, C or N contents % Cr,% Mo, % Nb, % Si, % Ti, % Ni, % Mn, % C or % N of the cast steel melt,which are selectively present in each case for adjusting thecharacteristics of the steel strip, are in each case selected in such away that for the ratio Cr_(eq)/Ni_(eq) formed from the Cr equivalentCr_(eq) and the Ni equivalent Ni_(eq), the following applies:Cr_(eq)/Ni_(eq)≧1.7,wherein Cr_(eq)=% Cr+1.37% Mo+2% Nb+1.5% Si+3% Ti,

-   -   Ni_(eq)=% Ni+0.31% Mn+22% C+14% N+% Cu.

The invention is based on the realisation that a specific minimumcontent of H₂ in the atmosphere covering the melt pool has a beneficialeffect on the surface composition of the steel strip obtained if, at thesame time, a steel alloy is used, the alloy contents of which arecoordinated in such a way that the ratio formed from its Cr equivalentand its Ni equivalent is at least 1.7. The formulae specified forcalculating the Cr equivalent and Ni equivalent correspond to thosedetermined by Hammar and Svensson in “Solidification and Casting ofMetals”, The Metals Society, London, 1979, pages 401 to 410.

In addition to the chromium equivalent Cr_(eq), the nickel equivalentNi_(eq) is a characteristic providing information regarding thestructural contents of stainless steels of the type processed in themanner according to the invention. Nickel and chromium are present inthese types of steel in considerable mass contents. Ni is an austeniteformer, while Cr is a ferrite former.

The contents of the alloy elements specified in the formulae fordetermining the Cr and Ni equivalents may, of course, also be “0”, sonot each of the relevant elements necessarily has to be present in steelprocessed in the manner according to the invention. Examples of typicalsteel alloys that may be processed in the manner according to theinvention include, for example, the steels pertaining to Class AISI 304and comparable austenitic Cr/Ni steels. Ferritic high-grade steels andcarbon steels are, however, also suitable for the method according tothe invention, since in the conventional procedure for casting thesetypes of steels, too, scums and cracks appear at the surface of thestrip. These material defects may also reliably be controlled using themethod according to the invention on ferritic high-grade steels andcarbon steels.

The hydrogen present, according to the invention, in the atmosphereprotecting the melt pool with respect to the environment causes thebinding of oxygen introduced into the region of the melt pool, forexample, via the elements, which move during the casting operation, ofthe casting device or other inevitable non-sealed areas. The risk ofreoxidation of the melt is therefore effectively counteracted. Theentrapment of oxides (“scums”) in the surface of the cast strip is thusreduced to a minimum.

The presence of hydrogen in the atmosphere above the melt pool alsoassists the breaking-down of the nitrogen into its atomic components(N₂->2 N). In this (atomic) state, the nitrogen may be absorbed at thesurface and diffuse into the steel, which is alloyed in accordance withthe prescriptions according to the invention. This ensures homogeneoussolidification, which is reflected in good imprinting characteristics,and prevents crack formation. The formation of the surface cracks istherefore reliably prevented, without the surface of the walls enteringinto contact with the melt having to be configured in a particularmanner. In the case of the procedure according to the invention, thereis therefore no need, especially if a twin-roller is used, for complexmanufacture and maintenance processes, such as are inevitable in theprior art considered at the outset. Instead, the walls delimiting thecasting gap may be provided with a stochastic unevenness distribution,such as is conventionally produced by an abrasive blasting process (forexample, R_(a)>40 μm, preferably >60 μm, R_(z)>7 μm, preferably >10 μm).

Adjusting the hydrogen content of the atmosphere to at least 0.5 mol %reliably ensures that the effect according to the invention caused bythe presence of the hydrogen is achieved. In order also to ensure thatthe hydrogen does not react explosively with the ambient oxygen, thehydrogen content of the atmosphere may be limited to no greater than 7.5mol %.

If it should be found that the solubility of the nitrogen in the steelhas undesirable repercussions, the supply of nitrogen in the protectiveatmosphere above the melt pool may be reduced in that a noble gas,preferably argon, is added to the atmosphere. It is accordinglyexpedient to vary the nitrogen content in the protective atmospherewithin a range, the lower limit of which corresponds, if a third gas ispresent in the gas mixture, to 30 mol %, and the upper limit of whichcorresponds, in the absence of a third gas, to the remainder apart fromthe respective H₂ content.

A further important configuration of the invention provides that, forthe ratio Cr_(eq)/Ni_(eq), the following applies: Cr_(eq)/Ni_(eq)≧1.8.It has surprisingly been found that in the processing according to theinvention of steels for which this required ratio of the Cr equivalentto the Ni equivalent is adhered to, an optimal surface appearance may beensured. In the case of steels having a composition of this type, whichare held during the casting process, in the manner according to theinvention, under an atmosphere containing hydrogen, there is no longerheterogeneous shine. Instead, the observer sees a uniform, consistentsurface shine of the finished strip, which is no longer marred by spotsor comparable inhomogeneities such as grey mottling, streakiness orsimilar appearances. This may be accounted for by the fact that, in themode of manufacture according to the invention, the transfer of heatbetween the moving walls, delimiting the casting gap, and the melt isrendered uniform in that, as described above, any oxygen conveyed by therelevant walls is bound by the atmospheric hydrogen, and the nitrogen isin a state in which it may easily diffuse into the steel melt. Onleaving the casting gap, the steel, which enters into contact with thewalls, accordingly has in the region of its surface a uniform structure,which is a prerequisite for the formation of a uniform surface shine.

In addition to the above-described advantages of the invention, a basiceffect of the increase according to the invention in the ratioCr_(eq)/Ni_(eq) and the addition of hydrogen to the atmosphere above themelt pool is that the casting capacity rises significantly above thatwhich may be achieved using the conventional procedure. This increase incasting capacity is obtained even if the lower limit, specifiedaccording to the invention, for the ratio Cr_(eq)/Ni_(eq) is adhered toand hydrogen is added within the range specified according to theinvention, and may be further increased by increasing the value of theratio Cr_(eq)/Ni_(eq) and the content of hydrogen in the atmosphereabove the melt.

The effects achieved by the invention are obtained independently of therespective sulphur content of the processed melt. There is therefore noneed to restrict the application of the invention merely to specificsteel alloys. The invention will be described below in greater detailwith reference to a drawing illustrating embodiments. The figure is aschematic side view of a twin-roller.

The twin-roller 1, which is configured in a manner known per se and isused as a device for casting steel strips B, having a thickness from 1mm to 10 mm, from a steel melt, comprises two casting rollers 2, 3,which rotate in opposite directions during the casting operation anddelimit between them a rectangular casting gap 4 in the longitudinaldirection thereof. The circumferential surfaces of the casting rollers2, 3 form the walls, which move during the casting operation, of thecasting gap 4. The short sides of the casting gap 4 are sealed by sideplates (not shown).

The steel melt is cast from an overflow (also not shown) into thecasting gap 4 using a dip tube 5. Accumulated melt forms a melt pool 6above the casting gap 4.

The region above the melt pool 6 is sealed off with respect to theenvironment by an enclosure 7. Above the surface of the melt pool 6, thecomposition of the atmosphere A inside the enclosure 7 differs from thatof the open environment U outside the enclosure 7.

Casting tests were carried out on melts E11, E12, E13, E21, E22, E23 andV11, V12, V13 using the device 1. The tests carried out on the melts E11to E23 took place in the manner according to the invention, while thecasting tests V11 to V13 were used to demonstrate the results obtainedin the conventional mode of manufacture.

The compositions of the respectively cast steel melts E11 to E23 and V11to V13 are specified in Table 1. In the case of the melts V11 to V13,which were processed for the purposes of comparison, the ratioCr_(eq)/Ni_(eq) is less than 1.7, while the ratio Cr_(eq)/Ni_(eq) forthe steels E11 to E13 is between 1.7 and 1.8, and for the steels E21 toE23 is more than 1.8.

The melts were, in each case, cast to form strips of varying thickness.The N₂, Ar and H₂ content of the composition of the atmosphere A in theenclosure 7 above the melt pool 6 was, in each case, varied. Therelevant strip thicknesses and the further operating parameters arerecorded in Table 2.

Table 2 also contains the results of the assessment of the surfacecomposition of the strips produced from the melts V11 to V13 and E11 toE23. In the case of the comparative examples V1 to V3, it was found thatalthough varying the N₂ and Ar contents of the atmosphere A allowed theoccurrence of scums and cracks to be influenced, only the additionaccording to the invention of H₂ to the atmosphere A causes asignificant increase in the reliability with which a high-quality caststeel strip product, the visual appearance of which also meets strictrequirements, may be provided.

The tests that were carried out reveal that the casting capacity may besignificantly increased by applying the invention. It could thus bedemonstrated that an increase in the ratio Cr_(eq)/Ni_(eq) causes acorresponding rise in the casting capacity. Diagram 1 shows in graphform the relationship between the casting capacity and the ratioCr_(eq)/Ni_(eq).

LIST OF REFERENCE SIGNS

-   B Steel strip-   1 Twin-roller-   2, 3 Casting rollers-   4 Casting gap-   5 Dip tube-   6 Melt pool-   7 Enclosure-   A Atmosphere above the melt pool 6

U Environment TABLE 1 C Si Mn P S Cr Mo Ni Al B Co Cu N Nb TiCr_(eq)/Ni_(eq) V1 0.045 0.29 1.39 0.025 0.001 17.01 0.20 8.46 0.0030.0004 0.13 0.18 0.046 0.01 0.001 1.66 V2 0.048 0.39 0.41 0.026 0.00117.21 0.12 8.42 0.003 0.0004 0.13 0.20 0.064 0.01 0.002 1.68 V3 0.0450.30 1.25 0.026 0.003 17.14 0.22 8.40 0.003 0.0004 0.02 0.25 0.060 0.010.001 1.65 E11 0.038 0.36 1.04 0.022 0.001 18.00 0.52 8.93 0.003 0.00040.17 0.02 0.039 0.01 0.001 1.78 E12 0.042 0.44 1.26 0.025 0.001 18.280.28 8.52 0.003 0.0006 0.11 0.29 0.051 0.01 0.002 1.79 E13 0.050 0.461.27 0.025 0.013 18.10 0.30 8.63 0.003 0.0004 0.16 0.26 0.054 0.01 0.0071.73 E21 0.047 0.30 1.30 0.020 0.003 18.06 0.36 8.09 0.003 0.0004 0.170.22 0.054 0.01 0.009 1.81 E22 0.039 0.33 1.26 0.024 0.001 18.04 0.168.01 0.003 0.0004 0.09 0.16 0.057 0.01 0.001 1.84 E23 0.039 0.40 1.260.024 0.001 18.14 0.27 7.99 0.003 0.0004 0.02 0.16 0.050 0.01 0.010 1.89All data specified in % by weight, the remainder being iron andconventional impurities

TABLE 2 Composition of gas Roller mixture Surface defects on the Stripunevenness supplied strip thickness [μm] [mol %] HeterogeneousCr_(eq)/Ni_(eq) [mm] Rz Ra N₂ Ar H₂ Cracks Scums shine V11 <1.7 2.8 10020 53 47 0 None Some Distinct V12 2.7 92 19 18 77 5 None None DistinctV13 2.7 90 18 95 0 5 None None Distinct E11 ≧1.7 3.0 82 17 70 30 0 SomeSome Slight E12 3.0 82 17 15 80 5 Some None Slight E13 2.7 102 20 81 145 None None Slight E21 ≧1.8 2.9 86 18 48 52 0 Many Some None E22 2.7 7816 23 72 5 Some None None E23 3.0 78 16 85 10 5 None None None

1. A method for the production of cast steel strip in a continuousprocedure comprises casting a steel melt into a casting gap havinglongitudinal sides of which are formed by walls that move during thecasting process, to form the steel strip, and holding the steel melt,which is present above the casting gap in a melt pool, under anatmosphere containing nitrogen and hydrogen, wherein the hydrogencontent of the atmosphere is greater than 0 mol % to 10 mol %, and theCr, Mo, Nb, Si, Ti, Ni, Mn, C or N contents of the cast steel melt,which are selectively present in each case for adjusting thecharacteristics of the steel strip, are in each case selected in such away that for the ratio Cr_(eq)/Ni_(eq) formed from the Cr equivalentCr_(eq) and the Ni equivalent Ni_(eq), the following applies:Cr_(eq)/Ni_(eq)≧1.7, wherein Cr_(eq)=% Cr+1.37% Mo+2% Nb+1.5% Si+3% Ti,Ni_(eq)=% Ni+0.31% Mn+22% C+14% N+% Cu, % Cr=respective Cr content, %Mo=respective Mo content, % Nb=respective Nb content, % Si=respective Sicontent, % Ti=respective Ti content, % Ni=respective Ni content, %Mn=respective Mn content, % C=respective C content, % N=respective Ncontent.
 2. The method according to claim 1, wherein the casting gap isformed between two casting rollers, which rotate in opposite directions,are cooled during the casting operation and delimit the longitudinalsides of the casting gap.
 3. The method according to claim 1, whereinthe hydrogen content of the atmosphere is at least 0.5 mol %.
 4. Themethod according to claim 1, wherein the hydrogen content of theatmosphere is no greater than 7.5 mol %.
 5. The method according toclaim 1, wherein the atmosphere additionally contains a noble gas. 6.The method according to claim 5, wherein the noble gas is argon.
 7. Themethod according to claim 1, wherein the nitrogen content of theatmosphere is at least 30 mol %.
 8. The method according to claim 1,wherein for the ratio Cr_(eq)/Ni_(eq), the following applies:Cr_(eq)/Ni_(eq)≧1.8.
 9. The method according to claim 2, wherein thecasting rollers have a stochastic unevenness distribution.